The two primary approaches for coal refining for the purpose of converting coal to liquid fuels are called direct and indirect coal liquefaction. Direct coal liquefaction (DCL) reacts coal in a solvent with hydrogen at high temperature and pressure to produce liquid fuels. DCL was first developed in Germany in 1913 and used commercially in Germany between 1927 and 1945. However, after World War II, crude oil was widely available at reasonable prices and the implementation of coal liquefaction was therefore not commercially attractive. As a result, only a small quantity of liquid fuels sold today is produced using a coal liquefaction process.
Indirect coal liquefaction (ICL) involves first gasifying coal to produce a synthesis gas which contains principally carbon monoxide and hydrogen and thereafter catalytically processing the gas chemically into a variety of liquid fuels.
Where diesel type products are desired, ICL plus the Fischer-Tropsch process is preferably used to convert the synthesis gas. The ICL technology was commercially applied in the 1920-1940's in Germany and since the 1950's in South Africa. While commercially demonstrated, the ICL technologies are very complex, capital intensive, and have low thermal efficiencies compared to direct coal liquefaction.
An important DCL technology is the H-Coal Process which was invented by Hydrocarbon Research, Inc. and is generally described in U.S. Pat. Nos. 3,519,553 and 3,791,959. The H-Coal Process uses a single ebullated-bed reactor with a hydroconversion catalyst to convert coal to liquid fuels. The ebullated-bed reactor is unique in its ability to process solids containing streams in the presence of high activity hydrogenation catalyst particles. Product oil (400° F.+) was used to slurry the coal for feeding to the reactor. Coal liquefaction took place at temperatures of 800-875° F., and hydrogen partial pressures of 1500-2500 psia. With Illinois No. 6 coal, liquid yields of greater than 50 w % on DAF Coal were achieved during the multi-year year demonstration program at the 200 ton per day H-Coal Pilot Plant in Catlettsburg, Ky. The DCL technologies demonstrated commercial readiness, however, no commercial projects proceeded as oil prices fell and oil supplies increased.
In the 1980's and 1990's research continued at a smaller scale to improve the DCL technologies and reduce investments and operating costs. The Catalytic Two-Stage Liquefaction Process (H-CoalTS) was invented by Hydrocarbon Research, Inc., as described in U.S. Pat. Nos. 4,842,719, 4,874,506, and 4,879,021, to substantially increase the yield of distillate liquids from coal. For Illinois No. 6 bituminous coal, liquid yields were increased from 3 barrels per ton of DAF coal for the single stage H-Coal Process to about 5 barrels per ton of DAF coal for the H-CoalTS Process. This was achieved by dissolving the coal feed at mild conditions while simultaneously hydrogenating the coal recycle solvent and coal liquids produced at temperatures from 600-800° F., hydrogen partial pressures of 1500-2500 psia in the presence of a hydrogenation catalyst.
In the H-CoalTS Process, the unreacted coal from the initial stage is then fed to a direct-coupled second stage reactor operating at higher temperatures of approximately 800-850° F. and at similar pressures (1500-2500 psia) with a hydroconversion catalyst, to achieve maximum coal conversion and high distillate liquid yields. The hydrogenation catalyst used for the single-stage and two-stage processes deactivates at these reactor conditions due to the deposition of coke and also soluble metals from the coal feed if present.
Whether in a direct or indirect coal liquefaction process, process fired heaters are collectively a major source of carbon dioxide emissions. In a typical direct coal liquefaction facility, for example, approximately fifty percent (50%) of the carbon dioxide emitted to the atmosphere originates from process fired heaters. These fired heaters combust natural gas, refinery fuel gas or liquid fuel oil with air and emit a flue gas which is relatively dilute (15 V % or less) in CO2 with nitrogen present as the primary gas. Although technology exists for capturing the dilute CO2 from this stream, it is highly capital intensive and not practiced commercially.
Another method for recovering CO2 from the fired heater flue gas involves feeding of purified oxygen from an air separation plant to the fired heaters. This significantly increases the CO2 concentration in the fired heater flue gas where it is more economic to recover CO2 after removal of sulfur species, particulates and water. However, this configuration requires a recycle of CO2 to the furnace to moderate the resulting temperature and also results in high investment and operating costs primarily related to the air separation plant and is not economically practical. Additionally, separate CO2 capture facilities may be needed for each fired heater.
As a result of the need for an efficient and economical technique for reducing the CO2 emissions in these CBTL (Coal and/or Biomass to Liquids) facilities, applicant herein discloses a novel process to eliminate the need for post combustion CO2 capture from fired heaters (at atmospheric pressure and in dilute phase) by adding a steam methane reformer (SMR) unit to the complex to produce additional process, hydrogen as well as the hydrogen fuel needed for the process fired heaters.
Relative to existing technologies and process configurations for CO2 capture, the invention will have significantly improved economics and will result in the capture in excess of 70-percent of the CO2 produced in a DCL facility. The original concept was developed for DCL, although the invention can also be applied to ICL as well as for coal, biomass, and petroleum co-processing. In the text, the term H-CoalTS is used for any liquefaction process in one, two or more steps for treating a carbonaceous material.
In a DCL facility, the unconverted coal and vacuum residue portion of the coal liquefaction products are typically routed to a gasifier to produce a portion of the required hydrogen. Hydrogen is required for both the coal liquefaction reactors and in the secondary hydrotreating/hydrocracking steps. In a DCL facility that does not utilize the Applicant's invention, the remaining required hydrogen is typically produced by the gasification of additional coal or by the steam methane reforming (SMR) of the produced fuel gas and/or natural gas.
When it is necessary to capture CO2 to prevent its release to the atmosphere, several commercially available technologies such as Selexol and Rectisol or amine solutions can be used to recover CO2 from the gasification or SMR synthesis gases. However, significant CO2 emissions remain from the plant fired heaters and steam boiler where tail gas, plant fuel gas, imported natural gas or fuel oil are used as fuel. In a typical DCL plant processing Illinois No. 6 bituminous coal, approximately 50% of the CO2 emissions originate from fired heaters with the remaining originating from the gasification and SMR plants. By adding CO2 capture to the hydrogen production plants (gasification and SMR), only about 50 to 70% of the total CO2 produced in the facility can be captured.
Applicant's invention represents a dramatic improvement over the conventional designs as described above. Applicant's adds additional SMR plant capacity or a new SMR Unit to the complex designed to produce additional hydrogen which can be thereafter utilized for the process consumers, as required for the plant fired heaters (including the SMR furnace), and for the production of plant steam. The plant light ends (C1, C2, etc.), which are normally utilized as fuel gas streams are the primary feeds to the SMR Unit along with the tail gas purge from the Gasifier Pressure Swing Absorber (PSA) unit.
The SMR synthetic gas (syngas) is shifted using a water gas shift unit and the CO2 thereafter recovered utilizing an absorber. The tail gas from the SMR PSA is recycled to the SMR reactor with a small purge used for the SMR furnace fuel. All flue gases from the plant are greatly reduced in CO2 content or nearly CO2 free since high purity hydrogen is utilized as fuel. The design of the SMR is optimized so that the required hydrogen is produced while the plant steam requirements are also balanced.
The invention is applied where CO2 capture is required to meet environmental requirements. CO2 capture technology is initially added to the hydrogen production process (gasification and/or SMR). Next, the feeds to the SMR (reactor and furnace) and the type of fuel to the plant fired heaters are modified to minimize the release of CO2 to the atmosphere. The invention thus comprises the following features: 1) inclusion of a SMR Unit to produce process hydrogen as well as the fuel for plant fired heaters, 2) routing of the carbon-rich gasifier PSA tail gas to the SMR feed, 3) inclusion of CO2 recovery in both the gasification and SMR Units, 4) recycle of nearly all the SMR PSA tail gas to the SMR reactor feed with a small purge to the SMR furnace to reduce inerts such as nitrogen, argon in the hydrogen product, and 5) optimization of the SMR design to produce the required hydrogen while also balancing the plant steam requirements.